1. Field of the Invention
The present invention relates to apparatus for the simultaneous measurement of both the linear and angular displacements of a plane mirror. More particularly, the invention relates to optical apparatus which is useful for high accuracy linear and angular displacement metrology using interferometry.
2. The Prior Art
High accuracy linear and angular displacement measurements are required in the machine tool industry and in the semi-conductor industry. Linear displacement is commonly measured with an interferometer. Angular displacement is commonly measured with either an interferometer or an autocollimator.
There are numerous interferometer configurations which can be used to measure the linear displacement of a plane mirror. The plane mirror interferometer and the differential plane mirror are the two most common, see for example S. J. Bennett, "A Doubled-Passed Michelson Interferometer," Opt. Comm. 4, pp. 428-430, 1972, R. R. Baldwin and G. J. Siddall, "A Double-Pass Attachment for the Linear and Plane Interferometer," Proc. SPIE, Vol. 480, pp. 78-83 (May 1984), and G. E. Sommargren, U.S. Pat. No. 4,693,605, issued Sept. 15, 1987.
Sommargren, U.S. Pat. No. 4,717,250, issued Jan. 5, 1988, describes an angular displacement measuring interferometer.
It is possible to measure simultaneously the linear and angular displacements of a plane mirror by using either (1) two linear displacement interferometers offset from each other, or (2) a linear displacement interferometer and an angular displacement interferometer or an autocollimator.
However, using two devices, one to measure linear displacement and the second to measure either linear displacement or angular displacement, has the following disadvantages: (1) complexity because two devices must be installed and aligned, and (2) considerable space is needed thereby requiring that the size of the mirror being measured be increased, especially if it moves in a direction in the plane of the mirror.
The present invention retains the preferred characteristics of both the linear displacement interferometer and the angular displacement interferometer while avoiding the serious limitations of using two of these devices. In the present invention, linear and angular displacements of a plane mirror are measured in a single, compact dual interferometer. The improvements of the present invention thusly overcome the disadvantages of the prior art and allow the high accuracy, simultaneous measurement of both linear and angular displacements of a plane mirror, i.e., to a small fraction of a micrometer and of an arc second, respectively, required for precision high speed X-Y stages.